In recent years RFID (Radio Frequency Identification) or the like for reading and writing information by radio communication has widely been used as a contactless automatic identification technique. With the RFID contactless data communication is performed by the use of radio waves (electromagnetic waves), for example, between a tag (RFID tag or the like) including a semiconductor memory and a reader-writer device for reading out data from and writing data to the semiconductor memory of the tag. With the RFID data communication is performed, for example, in the following way. The reader-writer device transmits radio waves to the RFID tag, information in the RFID tag is transmitted again to the reader-writer device by the use of radio waves, and the reader-writer device reads the information in the RFID tag transmitted.
A radio frequency used in the RFID belongs to the UHF band. A frequency of about 868 MHz is used in the EU (European Union), a frequency of about 915 MHz is used in the US (United States of America), and a frequency, of about 953 MHz is used in JP (Japan). Usually communication distance which can be realized in the case of using one RFID tag is about 3 to 5 m, but this distance depends on the minimum operating electric power of a chip used in the RFID tag.
FIG. 25 illustrates the structure of a RFID antenna. With the RFID, for example, a RFID antenna 900 illustrated in FIG. 25 is connected to a reader-writer device to perform communication between the reader-writer device and a RFID tag. With the RFID antenna 900, a patch antenna 902 for outputting radio waves is placed on an antenna plate 901 which is, for example, about 20×20 cm in size.
In addition, the following are known as a technique for transmitting and receiving information by the RFID.
Japanese Laid-open Patent Publication No. 2005-Japanese Laid-open Patent Publication No. 2008-123231
For example, however, if a plurality of RFID tags are adjacent to one another and therefore a plurality of RFID antennas are adjacent to one another, interference may occur. This causes a fall in the gain of an antenna or a change in impedance in a RFID tag. As a result, the state of matching between a chip and the antenna in the RFID tag may get worse, resulting in a shorter communication distance.
For example, it is assumed that papers or the like on which RFID tags are stuck are arranged at intervals of 1 to 2 mm. Even if the output of the reader-writer device is increased (to about 1 W, for example) and the RFID tags are brought as close to the reader-writer device as possible, there are cases where communication cannot be performed with any RFID tags stuck on the papers. If the output of the reader-writer device is limited to a range (about 10 dBm (10 mW), for example) in which legal restrictions are not imposed, it is even more difficult to perform communication. As stated above, when RFID tags stuck on papers managed by the RFID go into a state in which they are close to one another (they are arranged at intervals of several millimeters, for example), there are cases where communication with the RFID tags becomes difficult.
In addition, with the above patch antenna 902 bandwidth which can usually be used is about 20 MHz. Accordingly, with a reader-writer device used in the EU, the US, or JP, a dedicated patch antenna corresponding to a RFID tag frequency specified in the EU, US, or JP standards is needed. Some RFID tags can accommodate all of the RFID tag frequencies specified in the EU, US, and JP standards. However, such RFID tags may be, for example, about 100×20 mm in size. This size does not make handling easy. Such RFID tags may not be very practical at the time of attaching, depending on the size of articles to be managed. Furthermore, when such RFID tags are close to one another, their wide band characteristics are lost.